1. Field of the Invention
The invention relates to the field of serial transceivers, and in particular, to speed negotiation between transceivers having different characteristics.
2. Background Information
A number of different speeds and standards are currently used and being developed in serial communications systems, for example, fiber optical communication links. Serializer/Deserializer (SerDes) modules are used in digital communications system transceivers to serialize and deserialize data which is sent in serial form over a communications medium.
Existing transceivers, also called legacy transceivers, have generally been designed to operate at a different and lower speed than the newly emerging transceivers. It is desirable to be able to continue to use the legacy transceivers in existing systems, but also be able to connect them with other systems having the faster emerging transceivers. A user may want to upgrade a system to the faster transceivers in stages, and therefore it would be desirable to be able to continue using the legacy transceivers while upgrading.
However, currently, problems may be encountered in a link having two or more transceivers designed for different operating speed standards. Generally, a link having transceivers with different operating speeds can operate consistently only at the speed of the lowest speed transceiver. Therefore, the higher speed devices need to be able to operate at the lower speed.
Further, it would be advantageous if each transceiver could automatically detect the respective speeds of the other transceiver to which it is connected in a point-to-point link, automatically determine when a speed adjustment is necessary, and automatically make the speed adjustment.
One proposal which was discussed at a meeting of the Fibre Channel Optical Working Group in the Fall of 1999, was to encode and detect different baud rates using a lights on, lights off sequence, the rate of ‘flashing’ being related to the baud rate. For example, a 1 Hz flashing rate would mean a 1 GHz baud rate. However, so far no standard has been adopted in this regard.
Therefore, a need exists for an arrangement to implement an automatic baud rate negotiation between transceivers having different baud rate operating characteristics.
Along with automatic baud rate detection and adjustment, there is also a need to select and adjust to the appropriate speed-dependent (baud rate dependent) filtering because different link baud rate/speed standards require different transceiver bandwidths. Solutions to the bandwidth adjustment problem are described in copending application attorney docket ROC920000029 (IBM-141), application Ser. No. 09/574,239, filed May 19, 2000, entitled “SWITCHABLE-BANDWIDTH OPTICAL RECEIVER” and assigned to the same assignee as the present application. A number of different ways in which receiver bandwidth can be adjusted are disclosed therein.
Known Fibre Channel standard optical transceivers have control input/output signals (I/O's) known as Transmit Disable (TxDsbl) and Receive Loss of Signal (RxLOS). These signals are presently used to disable a transmission and indicate the loss of a received signal, respectively. These signals may be used diagnostically to detect and/or indicate problems in a communications link, or for power conservation reasons, for example.